1. Field of the Invention
The present invention relates to a microlens structure and fabrication method thereof. More particularly, the present invention relates to a microlens structures formed in high index materials using photolithography, gray scale masks, and reaction ion etching, the structures generally having two anamorphic surfaces on opposing ends of a single substrate, and fabrication methods thereof.
2. Discussion of Conventional Art
FIGS. 1A-1B and 1C-1D respectively illustrate a laser beam 1 output from an ideal laser beam diode (hereinafter, an ideal laser beam), and a laser beam 2 output from a laser diode. As shown in FIGS. 1A-1B, an ideal laser beam is a highly collimated, circular beam, with a gaussian intensity profile. By contrast, as shown in FIGS. 1C-1D, a laser beam generated by a laser diode is a non-collimated, elliptical aberrations. As such, the laser beam generated by a laser diode diverges by different amounts in orthogonal planes. To generate a laser beam having characteristics more closely resembling the characteristics of an ideal laser beam, the output of a laser diode must therefore be circularized (e.g., changed from its elliptical shape to a circular shape) and collimated. However, conventional systems for collimating and circularizing laser diode outputs experience at least three types of problems.
First, conventional systems require multiple separate elements. Specifically, conventional systems use a combination of two or more separate simple anamorphic elements such as prisms and cylindrical elements, combined with rotationally symmetric elements, to perform circularization and collimation of laser beams. For instance, as shown in FIG. 2, a conventional laser assembly includes a laser diode 21 followed by a three-part collimating lens 22, a cylindrical lens 23, and multiple anamorphic prisms 24. Therefore, because conventional systems require several different anamorphic elements, they are expensive to manufacture and difficult to align.
Second, simple diffractive systems cannot provide the high optical powers required for collimating widely divergent laser beams, such as those generated by laser diodes. For instance, although diffractive optics are capable of achieving non-symmetric (anamorphic) beam shaping, their usefulness in correcting laser diodes is limited by fabrication constraints. Specifically, diffractive zone widths of less than approximately three (3) microns are required in order to collimate the wide divergence angles that are characteristic of laser beams generated by laser diodes. Yet, it is nearly impossible to fabricate diffractive optics with zone widths this small using conventional fabrication techniques including gray scale mask technology.
Third, simple refractive optics having anamorphic surface profiles suitable for collimating laser diode outputs are difficult to produce reliably. That is, although refractive optics typically achieve the optical powers required for collimation of a laser beam output, it is difficult to manufacture refractive optics with general anamorphic surfaces, i.e. different curvatures in orthogonal planes. One conventional method of manufacturing anamorphic refractive elements is to melt and stretch optical fibers. However, melting and stretching an optical fiber in this manner causes the design and fabrication process to become highly empirical and sensitive to numerous coupled process parameters (e.g., temperature and temperature distribution, fiber diameter, glass type, stress and strain, etc.). Another problem is that the resulting microlens is typically so small that alignment with the laser diode becomes very difficult.
Larger diameter refractive elements may be used to avoid alignment problems inherent in the melting and stretching process described above. Rather than melting and stretching, the surface of these elements may be shaped using conventional grinding and polishing techniques. However, when the conventional method of grinding and polishing is used to achieve a desired surface shape for the surface of refractive elements, the shape of those surfaces is limited to rotationally symmetric or simple cylindrical surface profiles. Arbitrary anamorphic surface profiles, such as saddle shapes, which are useful in collimating laser diode outputs, are therefore difficult to achieve with conventional grinding and polishing techniques.
New state-of-the-art diamond turning machines are capable of achieving bilaterally symmetric anamorphic profiles, but these are generally used for fabricating plastic molding tooling, not optical elements. Furthermore, diamond turning generates a fine periodic groove structure which must be removed with a post-polishing process to prevent scattering in the visible spectrum.
Moreover, conventional systems for collimating and circularizing laser diode outputs are problematic in at least three respects. First, conventional systems require multiple separate elements. Second, due to limitations and diffractive zone widths, conventional diffractive optics are not particularly well-suited for collimating laser diode outputs. Third, refractive optics having anamorphic surface profiles suitable for collimating laser diode outputs are difficult to reliably produce.